The right subclavian artery arises from the brachiocephalic
artery, behind the right sternoclavicular
joint. It passes upward and laterally as a gentle curve behind the scalenus
anterior muscle, and at the outer border of the first rib becomes continuous
with the axillary arch.

The left subclavian artery arises from the posterior part
of the arch of the aorta, a little behind
the left common carotid artery. It
ascends vertically to the root of the neck, opposite the left sternoclavicular
joint, and then arches laterally in a manner similar to that of the right
subclavian artery. Like the common carotid artery, it has no branches on
the thorax.

For descriptive purposes, the artery is divided into three parts. The first
part extends from the origin of the vessel to the medial margin of the scalenus
anterior; the second part lies behind that muscle; and the third part extends
from its lateral border to the outer border of the first rib.

First part of the subclavian

Owing to the difference of origin, the relations of the first portion of
the subclavian artery are not the same on the two sides of the body.

The first part of the right subclavian extends obliquely upwards and laterally,
and at its termination at the medial margin of the scalenous anterior it
has reached a point about half an inch above the level of the clavicle.
It is placed very deeply. Anteriorly, it is covered by the skin, superficial
fascia, platysma, deep fascia, and
three muscular strata – the sternomastoid,
sternohyoid, and sternothyroid. Two veins and some nerves are in front of
it: at the medial margin of the scalenus anterior it is crossed by the internal
jugular and vertebral vein. The nerves which cross in front of it are the
vagus, a loop from the sympathetic (ansa
subclavia), and, sometimes, cardiac branches of the vagus and sympathetic
as they run into the thorax. The common carotid artery also is in front
of it, near its origin.

The cervical plexus is both below and behind the artery, separated from
it by a thin fibrous sheet called the suprapleural membrane. At the lower
margin of the artery, the vagus gives off its recurrent laryngeal branch,
which hooks round below it and ascends behind it.

The first part of the left subclavian ascends almost vertically from the
aortic arch, and, when it reaches the root of the neck, it passes upwards
and laterally to the medial margin of the scalenus anterior. The relations
of the cervical part are slightly different from those on the right side.
The same fascial and muscular layers, and the same nerves and veins, are
in front of it. But in the rest of the neck, the veins and nerves of both
sides trend to the right; on the left side, therefore, they are more or
less parallel to the part of the subclavian artery that ascends behind the
sternoclavicular joint. Three additional relations are established –
the phrenic nerve and the thoracic duct descend in front of it near the
scalenus anterior; and the left innominate vein is in front of it, behind
the joint.

The left recurrent nerve hooks round the arch of the aorta, and lies to
the medial side of the subclavian artery.

Second part

The second portion of the subclavian artery forms the highest part or summit
of the arch, and rises above the level of the clavicle for half an inch
to an inch.

In that part of its course the vessel has not so many superficial relations.
Anteriorly, it is covered by (1) skin; (2) superficial fascia and platysma;
(3) deep fascia; (4) clavicular head of the sternomastoid; (5) scalenus
anterior. On the right side the phrenic nerve also is an anterior relation,
but it is separated from the artery by the medial margin of the scalenus
anterior. Posteriorly and inferiorly, the vessel is in relation with the
pleura – the suprapleural membrane intervening.